Continuous casting is an industrial manufacturing process wherein a metallic material in the liquid state, for example steel, is poured by gravity from a ladle into a tundish and from this into a continuous casting mould. As known, the mould of a continuous casting plant comprises an open bottom and side walls preferably but not exclusively made of copper, which, during operation of the plant, are constantly cooled preferably but not exclusively with water.
Thanks to the presence of a cooling system, the liquid metal which contacts the side walls of the mould is solidified thus forming a slab having a solidified “shell” around a “liquid core”. The shell provides the slab with a degree of stability suitable to allow its descent through a plurality of rollers arranged downstream of the mould, which preferably but not exclusively define an arc-shaped path the radius of which is a few meters long, wherein the solidification process of the slab continues. Once reached an horizontal position, the slab can be cut to a specific size or machined e.g. by direct rolling without solution of continuity in order to obtain a series of finished products such as sheets and strips. The latter process is also known as “cast-rolling”.
Plants for the manufacturing of slabs obtained by continuous casting are disclosed, for example, in the European patents EP 0415987, EP 0925132, EP 0946316 and EP 1011896 and in the international publication WO 2004/026497, all in the applicant's name, which relate in particular to the manufacturing of steel strips.
It is known that during a continuous casting process the mould is oscillated in a vertical direction, i.e. along the casting direction, in order to prevent solidified metal material from adhering to the copper side walls of the mould and to allow the supply of a lubricating medium that can reduce friction forces therebetween. The oscillation of the mould in the vertical direction preferably but not exclusively follows a sinusoidal law of motion.
For this purpose, the mould is generally mounted on a supporting and oscillating device comprising at least one support to which a servomechanism, such as a hydraulic jack, is connected so as to allow it to oscillate vertically. The support comprises in particular a fixed assembly restrained to a frame in turn mounted on a foundation, as well as a movable assembly slidably restrained to the fixed assembly along the vertical direction. The mould is mounted on the movable assembly, so that it can be moved vertically therewith. The movable assembly is connected to the servomechanism, therefore the total mass subjected to oscillatory movements includes the mass of the mould, the mass of the movable assembly of the support and the mass of the cooling fluid contained therein.
Preferably, but not exclusively, the supporting device comprises a pair of supports arranged symmetrically at the sides of the mould. In this case, the servomechanisms associated to the supports are properly coordinated with each other so as to generate on the supports of the mould oscillations of equal magnitude and phase.
The enormous technical and technological progress in the field of continuous casting plants allows to achieve a higher and higher “mass flows”, i.e. to increase the amount of steel per unit time coming out from the continuous casting. This involves the use of more and more powerful cooling systems for the moulds, which require high working pressures of the cooling fluid, for example in the order of 20 bar or higher, and high flow rates, which result in supply pipes having larger and larger cross-sections.
The cooling fluid, for example water, is supplied to the mould through channels formed in the supports of the oscillating device, and in particular in the movable assembly of each support. These channels generally extend in a vertical direction, so as to allow the connection of the pipes that supply the cooling fluid below the movable assembly. During the circulation of the cooling fluid, the combined effect of high operating pressures and large cross-sections of the channels generates hydraulic forces having a magnitude comparable to that of other forces normally acting on the mould during the operation of a continuous casting plant, in particular inertia forces related to the mass of the mould and pulsating forces generated by the servomechanism that causes the mould to oscillate. The hydraulic forces generated by in- or outflows of the cooling fluid tend in particular to lift the mould and its supports, thus being involved in the dynamic balance together with the pulsating forces intended to oscillate them. Therefore, the servomechanism must be designed by taking into account this dynamic balance of the forces, which results in solutions the construction and operation of which are not always satisfactory.
Another problem of known supporting and oscillating devices for continuous casting moulds is that oscillations imposed by the servomechanism to the elastic elements that hydraulically connect fixed pipes, which are generally arranged vertically upstream of the supporting device of the mould, and the movable assembly of the single support, generate pressure fluctuations in the channels formed in the supports and in the cooling circuit of the mould, thus altering the flow rate of the cooling fluid over time and potentially causing pulsating vaporization phenomena. This reduces heat exchange between metal and mould and thus penalizes the solidification process of the slab. A reduced heat exchange can also result in the formation of cracks in the copper side walls of the mould in contact with the metal passing therethrough, as well as thermal fatigue phenomena.
In order to solve this problem it is known to use hydropneumatic accumulators arranged along the branches of the cooling circuit of the mould. However, the use of hydropneumatic accumulators is problematic, because of their overall dimensions. Furthermore, in order to effectively reduce pressure pulsations that disturb the flow of the cooling fluid, hydropneumatic accumulators must be designed for specific frequency ranges and set at defined pressure levels, thus not being able to properly operate when the pressure of the cooling fluid varies e.g. at the discharge of the mould in function of its flow rate.